As the art moves towards smaller higher power integrated circuits such as SRAMs, heat transfer from the integrated circuit package (IC package) becomes increasingly difficult and more important. As used herein, the term "IC package" includes the heat generating integrated circuit as well as the packaging surrounding the integrated circuit.
One conventional technique to remove heat from an IC package is to employ a finned heat sink which is placed in thermal contact with the IC package. In this manner, heat generated by the IC package is conducted to the heat sink and then dissipated to the ambient environment.
Of importance, the heat sink must be reliably attached to the IC package in a manner which does not undesirably stress or damage the IC package or the circuit board to which the IC package is connected. One conventional technique is to employ a thermally conductive adhesive which bonds the heat sink to the IC package. However, thermally conductive adhesives do not adhere well to plastic IC packages resulting in an unacceptably high incidence of bond failure between the plastic IC package and the heat sink. Further, once the heat sink is bonded with the thermally conductive adhesive, it is difficult to remove the heat sink from the IC package without causing damage to the IC package, the heat sink or the circuit board. Yet, it is desirable to have a removable heat sink to readily allow chip repair, rework and/or replacement. Accordingly, it is desirable to avoid the use of thermally conductive adhesives altogether.
One prior art removable heat sink uses clips and/or fasteners to attach the heat sink directly to the IC package. However, when attached in this manner, the heat sink exerts undue force on the IC package which can damage and ultimately destroy the IC package. To avoid this problem, other removable heat sinks are attached directly to the circuit board to which the IC package is connected.
FIG. 1 is a side view of an electronic device 8 which includes a heat sink 10 directly attached by post type fastening members 12A to a circuit board 14 in accordance with the prior art. Located between heat sink 10 and circuit board 14 is an IC package 16 which generates heat during use. IC package 16 is typically electrically connected to circuit board 14 by one or more circuit interconnections, e.g., solder, which are not illustrated in FIG. 1 for purposes of clarity. Fastening members 12A urge heat sink 10 towards circuit board 14 and down onto IC package 16 to make the thermal contact between heat sink 10 and IC package 16.
Although providing the force necessary to make the thermal contact between heat sink 10 and IC package 16, fastening members 12A causes heat sink 10 to press unevenly on IC package 16. In particular, IC package 16 acts as a pivot between heat sink 10 and circuit board 14 so that end 10A of heat sink 10 is urged away from end 14A of circuit board 14 as indicated by arrows 18. This causes the force exerted by heat sink 10 on to IC package 16 to be greater at side 16A of IC package 16 than at side 16B. This uneven force distribution can damage and even crack IC package 16. Further, this uneven force distribution can create a gap between side 16B and heat sink 10 resulting in poor heat transfer between IC package 16 and heat sink 10. Alternatively, or in addition to, this uneven force distribution can cause circuit interconnection failure near side 16B of IC package 16. As those skilled in the art understand, these conditions can ultimately cause failure of device 8.
To avoid these drawbacks, it has become known in the art to attach both sides of heat sink 10 to circuit board 14. As an example, second post type fastening members 12B illustrated in dashed lines in FIG. 1 can be employed. This tends to equalize the force exerted by heat sink 10 on both sides 16A and 16B of IC package 16. However, this also causes the ends 14A, 14B of circuit board 14 to be pulled up by fastening members 12B, 12A, respectively, relative to the die attach region 14C of circuit board 14 to which IC package 16 is attached. This bending force, indicated by arrows 20, causes warpage of circuit board 14 such that circuit board 14 is displaced to a position 22. Over time, this warpage can cause device 8 to fail, e.g., from failure of circuit interconnections between IC package 16 and circuit board 14.
Accordingly, the art needs a heat sink assembly which results in even force application to the IC package and avoids circuit board warpage.